I don’t know about the UK but in the US we are in an “annoy the seller” type of market right now, after having enjoyed a “screw the buyer” market for a few years.
Yeah, no kidding. Its part of the process here. Now that the market has softened, the seller can take it or leave it.
What goes wrong with water heaters? My gas water heater is probably original to the house, so it’s 22 years old. Maybe. They seem like such simple devices compared to the HVAC system. I’ve always planned on replacing my water heater with an on-demand system when the time came, but is that time imminent now??
They can get a build up of hardness or other gunk that insulates the water from being heated efficiently or takes up enough space that you run out of water sooner then it’s rating.
They can spring leaks. Usually it is minor at first, you’d see a bit of water on the floor around the water heater. Every once in a while they leak spectacularly and you end up with lots of water in the basement.
Since your considering an on demand system I’d look into it sooner rather then later. Factors like the size of the gas line will effect your ability to do so. They price of the average unit is not something you want to be spending in an emergency when the hot water tank goes.
How much does it cost to have someone test all these things for the efficiency? The home inspector said that he will only certify that it work up to the point of the inspection. He even suggested getting a credit of some sort because they aren’t top brands. Will the inspector be able to give a good estimate of how long these things will last? I’ve changed a hot water heater once in my life…once. I will never go through that process again. It took three big dudes and a blow torch, not to mention so various other wrenches. Oh, and a typhoon amount of water.
I’ve never heard of anyone doing efficiency testing on residential HVAC equipment, but I imagine it’s possible. Generally, you just look at the stated ratings for the units and then derate it for condition.
I would not expect a home inspector to be able to give an informed opinion about how long equipment might last. It might even be unethical for them to do so. It’s also a tough thing to predict with any degree of accuracy. I know of a fair number of signs that would point to an imminent failure, but I’ve also seen a good number of failures without any of the usual signs. You could ask the inspector if he (or she) observed any such signs, but unless he is an experienced HVAC person I wouldn’t expect him to have noticed them.
For the furnace, as boytyperanma said, it’s hard to tell without being there, but it may be something as simple as a weak thermocouple. A new thermocouple costs less than $10 and takes about 15 minutes to install. A weak or mis-directed pilot flame will give similar symptoms.
I am NOT a plumber, and I’ve replaced at least 50 water heaters. After the first few times it gets to be amazingly easy*, but I’ve never once enjoyed the experience. 
*Usually - some of 'em are a gold-plated bastard to get in and out.
My parents had a water heater that lasted forty years. One of the mottos of environmentalists is “Use it up and wear it out.” (I think the saying came from the Depression.)
You don’t have to present yourself in a total panic to a salesman or a plumber when the WH goes out. You may have to heat some water on the stove for your bath for a couple of days while you comparison shop.
You might be surprised at how well you can make do for a couple of days when conveniences give out. I wouldn’t dream of replacing something that is still working!
That does have a downside. My stove top is Harvest Gold.
You may want to think about replacing it this year. There is a federal tax credit for various energy saving home improvements made before Dec. 31, 2007. There are also a lot of state and local incentives for various things. The DSIRE website is a database about the different programs and where to find out more.
As for the OP, I took possession of my house at the beginning of December. I had a 26 year old furnace, 140,000 BTU, and an 11 year old central AC unit. December was extremely warm here, and the thermostat was set at 55 because I was having the entire house rewired and not living here. My first gas bill, for 2 weeks was $218, actual readings. I got the bill in the beginning of January and decided to look at getting the furnace replaced. Because of my procrastination, and the slowness of one of the places where I was getting a quote (who of course ended up with the best quote and service reputation), the furnace (75,000 BTU, 93% AFUE) and AC (replaced with a 13 SEER heat pump) didn’t get replaced until just when the gas reading at the end of January were taken. It cost me $5400.
My January bill, still warm, thermostat still set at 55 to 60, was just about $300. My February gas bill, sub-freezing temperatures, programmable thermostat from 64 to 70 since I am living here now (plus using gas for water heating and cooking), $56. My electric has gone up because of the heat pump, but I don’t know how much yet they keep doing estimated bills. :dubious: But even if it goes up to $150, savings will pay pay off the expense in just a few years.
Not everyone will have those kinds of savings, but it is still worth looking into. I am hoping to get a tankless water heater later this year, but my gas feed from the street may be too small. 
Sorry, I mis-typed. My home inspector told me to find a HVAC person. I was wondering if a HVAC person could give me longevity estimates and efficiency ratings (but, I’m more concerned with the longevity). I can’t believe how much cash I’m shelling out for this place. I wonder if I can tap my home equity loan right off the bat to pay for this stuff?